1. Field of the Invention
This invention relates to solar energy collecting systems, and particularly to a new combination of an enclosure and an absorber or collector.
2. General Description of the Prior Art
In the past few years, and even before, many configurations of solar energy collectors have been proposed and some of them marketed. The principal problem today with solar energy collectors, either for heat or for electricity conversion, is, as with most products, providing an acceptable balance between cost, effectiveness, and durability. The fact that no single configuration has really captured the market is an indication that optimum designs are yet to appear. Considering the known types, perhaps the most common one for heat collection is the flat plate collector wherein a dark colored heat receiver is encased within an enclosure having a transparent or translucent face through which solar radiation directly impinges on the receiver and having a bottom side which is heavily insulated. Typically, the receiver contains a passageway or passageways through which a liquid, to be heated, is circulated. Depending upon the material through which the receiver is constructed, and thereby often its durability, a flat plate collector costs in the vicinity of $8.00 to $14.00 per square foot of active surface, with typical installation costs for a domestic hot water heater system running $800.00 to $2,000.00. This high cost is in part because of a typical requirement that there be a liquid-to-liquid heat exchanger to heat potable water and the use of a special fluid which flows between the heat receiver and the heat exchanger in order to avoid freezing or corrosion and deposits on the passageways of the heat receiver, which would render the receiver inoperative or ineffective after a relatively short period (in terms of the typical and expected life of a heating system, or even a hot water system, of 5 to 15 years). For electricity conversion, perhaps the most common one is a flat plate module arrangement of a number of photoelectric or photovoltaic cells, or solar cells (terms used interchangeable), encased within an enclosure having a transparent cover through which sunlight passes and impinged directly onto the solar cells. Typically, each solar cell is connected to electrical conductors which are brought to terminal connectors from which the electrical power may be taken. These flat plate solar cell modules, or photovoltaic arrays, are constructed such that the heat from the photoelectric cells may be removed from the back side, which are away from the sun, to keep the solar cells within the desired operating temperature range. Typically, a number of these modules are electrically connected together, as a photovoltaic system operational arrangement, to get the desired power at a desired voltage level. Currently, the cost of electricity using such module arrays is from $7.00 to 418.00 per watt. This high cost is due primarily to the expensive manufacturing processes to produce the photosensitive semi-conductor material for the solar cell. The delicate photoelectric cell semi-conductor must be protected from the effects of the environment to which it is exposed. A thin transparent covering is usually required to protect the solar cell surface from handling during manufacturing and assembly; beyond this, the amount of protective covering depends on the planned application. For space applications, sufficient covering must be used to protect the surface of the solar cells from micrometeorites; generally glass is used as the protective covering to minimize degradation from ultraviolet radiation. For terrestrial applications, the environment is more harsh due to dust, rain, hail, and other projectiles; glass is likewise preferred, but it is expensive and susceptible to breakage from impacts and thermal stresses. Polymer coverings are less expensive than glass and are more flexible but degrade in time due to ultraviolet radiation effects. A technique to reduce the cost and provide some protection to the solar cells is to utilize a photovoltaic system in conjunction with an enclosed concentrator device. For terrestrial applications, one such device is a linear trough-like arrangement in which the solar cells are located at the bottom with the sun-sensitive surface facing up toward the top of the trough, which is covered with a transparent material, such as glass. The sides slope up and outward to the top and are covered inside with a reflective material. In such an arrangement, the solar cells can be covered with a thin layer of glass as the trough top transparent covering protects them from the external environment. A portion of the sunlight entering the trough would strike the solar cells directly, and most of the remainder would strike the reflective inner sides and, in turn, be reflected and concentrated down onto the solar cells. Within limits, photoelectric cell power output is proportional to the amount of light striking it. Consequently, concentrators take advantage of this phenomenon getting more power out of the solar cells than that obtainable if the solar cells were in the usual flat plate arrangement. A significant problem with some types of known photovoltaic concentrators is that they must be adjusted in tilt for the sun's seasonal attitude and must track the sun throughout the day to be effective.
In an effort to solve some of the foregoing problems in collecting solar energy to heat water and to directly convert to electricity, some design improvements and research have been made. As an example, heretofore, it has been proposed that where the object is to heat water, a potable hot water tank itself be encased in a heat receiving enclosure, and that in addition to utilizing direct radiation from the sun, some reflected radiation be captured and furnished to the tank. One such system is illustrated in the September 1976 issue of "Popular Science" magazine, starting on page 101. This system employs an elongated tank in an enclosure with an elongated front and with two of the sides forming a light transmissive trapizoid. The back side, with a reflective inner surface, is parallel to the front side, and the top and bottom sides are perpendicular to the plane of the other sides and are heavily insulated. A difficulty with this configuration is that for optimum performance, it must be adjusted in attitude for the latitude of the location and as a function of the altitude (varying with seasons) of the sun. Preferably, some azimuth changes should be made through the day, i.e., tracking of the sun, for best solar energy capture.
To achieve direct conversion of sunlight into electricity utilizing photoelectric cells, solar cells, much research and development work has been done and is still being sponsored by the U.S. Department of Energy (DOE). The current mainstream effort by DOE is centered around their "Low-Cost Silicon Solar Array" (LSSA) Project. The prime emphasis of the LSSA Project is to develop low-cost silicon semi-conductor photoelectric cells and to assemble the cells into low-cost modules, each having a power output of approximately 10 to 15 watts. DOE is also doing some research and development work on photovoltaic solar concentrators.
Considering the foregoing, it is an object of this invention to overcome the stated problems, and particularly to provide an effective solar energy collector which may be used to directly heat potable water and/or provide an efficient photovoltaic system that is long-lasting, and is of a configuration which provides a substantial measure of angular compensation, enabling it to be constructed with a fixed orientation, and yet be of improved effectiveness despite significant variations in both azimuth and altitude (seasons and latitude) of the sun.